It's going to be a group so varied levels of fitness/experience, I'm hoping we'll be able to pull it off in 5-6 hours.
I wonder if anyone has a preferred route, or a good place about mid-way to stop, any other tips or advice...
My first thought on reading your post was "I wonder if "the" trail goes along part of that route?" And it does! You could do it partly on the Confederation Trail: https://goo.gl/maps/D1GEyeFpQmC2 The trail is ideal for biking with mountain bikes or cyclo-cross bikes (not so much for road bikes with narrow tires though). It's converted from the rail line that used to traverse the island and has a dirt/crushed gravel surface. Here is a streetview look at where you get onto the trail: https://goo.gl/maps/43H9eFgWovp
Krp has given you great info the Confederation trail is the way to go I have ridden some parts of the trail. You will avoid automotive traffic and have a safe way to travel. The trail from St Pete to Morell is one of the most beautiful section with water views along the way and bridges to stop and take it all in. Because of the stone red base of the trail your gears will attract a lot of dust and grime, make sure you bring supplies to clean and lubricant at the end of each day or so. You will have a lot of company on the trail with walkers and other bikes. You will find covered rest stops and picnic tables along the way and official tourist information centers. Being an old railbed the inclines are very gentle and easy to ride.
Been years since I was on the trail. Is it still gravel? That would sure make it a tougher ride.
Yea still gravel but packed very well in most places with some grass mixed in, not difficult at all to ride on.